Sadiq Khan: why I'm running the London Marathon

"Have you got an alibi?" The question I'd asked hundreds of times in my former life as a defence lawyer was now a question for me.

How could I wriggle out of the idea that I should run the London Marathon? I knew I'd need a good alibi. The idea had come from my friend, the journalist David Cohen. And my wife Saadiya was weighing in – I had to do something to balance my passion for Mars bars.

I tried a few lines: I am not a runner. I am not fit. I don't have the right kit. And 26.2 miles is an awful long way! There had been the odd jog around Tooting Common in my constituency in south London, fuelled by guilt about the Mars bars, but that's about it. Saadiya was unmoved and David was full of gentle reassurances.

The clincher was the one that has worked for thousands of London marathon runners past and present – a chance to help a good cause. And there's no doubt that the Dispossessed Fund is a brilliant one. Their work tackling poverty and deprivation across London is inspiring.

So on 2 February, I laced up my trainers and headed out on my first training run for the London Marathon 2014. The longest I can remember running is 10K – and that's a jog, mind you. Having started pretty late in the day, David has put me on a strict training regime. And on that first rainy morning I found it tough. Really tough. Getting into a steady stride in the drizzle took a while, but after about 5K or so it started to feel better. I relaxed, and running in a rhythm began to feel more natural. Several laps of the Common later I felt pretty pleased with having run a decent 12k – far further than I expected – and headed home for a bath (and, yes, a well-deserved Mars bar).

The smugness didn't last long – I can't describe the pain I was in the next day. Hobbling around parliament, trying to hide my limp from other MPs who I hadn't yet told about my new project. I learned my lesson the hard way: don't neglect warming down and stretching after a long run.

A few weeks on and, dare I say it, the training is going well. On a Sunday in late February I ran my first ever road race – David and I, along with my good friend and local GP Dr Tom Coffey, took part in the Hampton Court half marathon.

Meanwhile, each run is a little easier, and the stiffness the next day is more manageable. Another lesson I've learned: I need music to run to. I've found some good tracks, from Jay-Z to Paul Weller. More than once I've been caught attempting to sing along while running to an 80s classics.

But then there other times when running gives you a peacefulness and focus of mind that is an amazing escape from the Westminster bustle. My morning runs have become one of the only times I have during the week to reflect and take stock. And there are the obvious benefits. I feel fitter than I have in a very long time. And I am lighter than I have been for a while, too!

Three things have kept me pounding when my legs are screaming to stop. First, the huge number of projects supported by the Dispossessed Fund across London, from law centres to youth clubs, careers training to tackling gang culture. The charity has so far touched the lives of more than 100,000 Londoners, and knowing the money I hope to raise will go to help even more is a real boost to keep going.

Second, I am proud to be running in the same marathon that my Olympic hero Mo Farah will be competing in (and winning – come on Mo!).

And third, I am desperate to beat my good mate Ed Balls, the shadow chancellor, who is running the marathon for the third time in support of Whizz-Kidz and Action for Stammering Children. I've told Ed it will be a victory for youth over experience, plus I'll have the crowd on my side because I'm the home boy. Competitive, moi?